On toads and roads: The case of Spångavägen and Kyrksjölöten – tunnel mitigation is effective for spring migration but toads are still killed in large numbers

The nature reserve Kyrksjölöten in suburbian Stockholm is an important breeding area for common toad (Bufo bufo). The reserve is however surrounded by roads of different size, the largest being the municipal commuting road Spångavägen with 6,000-7,800 vehicles/day. Over winter, toads are distributed over a larger area, in particular on the opposite side of the commuting road. For many years, local observers have reported high numbers of amphibians killed along this road during spring migration. As a response, Stockholm municipality installed two ACO amphibian tunnels with permanent guiding fences over a 300 m road section. The permanent mitigation was installed in 2014, in connection with a road upgrading, but it was preceded by two springs (2012-2013) with temporary fencing and pitfall traps, when amphibians were counted along the fence and in traps, and manually translocated over the road. In 2015, we monitored tunnel usage and counted live and dead amphibians along the guiding fences (2x300 m) and roads (3925 m) surrounding the nature reserve, in order to assess the mitigation effectiveness in preventing road kill, maintaining connectivity and preserving the toad population. Tunnel usage was monitored with Froglife timelapse cameras, for one month covering peak migration. We used counts from the previous two springs as a reference. Counts along roads and guiding fences were done three evenings during peak migration time. As no count of road killed toads were done before fencing (i.e. not before 2012 when temporary fences were installed), we interpolated the pre-fencing number along the mitigated stretch based on adjacent, unmitigated road sections. Common toad was the totally dominant amphibian species (ca 98 % of all observations). Cameras showed toads moving in both directions, but the net movement was 866 toads towards the breeding area, which is a higher number than what was translocated in previous years (419 in 2012, 647 in 2013). Results from the counts of toads along the fenced road section suggested a 90-98 % decrease in live and dead toads on the road surface. Large numbers of toads were found on other parts of the commuting road and smaller roads surrounding the reserve, indicating that only part of the local toad population is approaching the breeding area over the fenced road section. We found peaks in road kills just outside the fenced section, suggesting some animals bypassing the fence ends. The lack of quantitative data on the local population and road kill numbers before mitigation limits the inference of the results, but we find it safe to conclude that the fencing has significantly reduced the number of road killed toads along this previous ”black stretch”, and that the tunnels allow the vast majority of the toads reaching the fence to safely pass to their breeding area. It remains unclear what proportion of the local population is protected by this measure and what are the effects on mortality and movements during summer and autumn migration in the opposite direction, especially for juveniles.

Amphibian tunnels, Camera, Common toad, Effectiveness, Road kill, Stockholm